Because the usable electromagnetic spectrum is a limited resource, governmental agencies regulate its use and exploitation. For example, in the U.S. the Federal Communication Commission (FCC) divides the usable electromagnetic spectrum into frequency ranges, or bands. Each band may be assigned to a specific function, or may be reserved for future use. Since only a few bands are allocated to a specific function, such as portable communication, it is important that the band be efficiently utilized.
Thus, the frequency bands assigned to wireless communications should accommodate many wireless users. To allow each frequency band to accommodate so many users, a wireless communication device typically uses a reference oscillator to accurately find the carrier frequency used by its particular base station. In a particular use, the wireless device is a mobile handset configured to communicate with one or more base stations.
To find the carrier frequency, reference oscillators operating in a wireless communications device typically generate a highly accurate frequency reference. Because low-cost oscillators are prone to substantial initial error, large individual variations, and degraded performance over time, they have not been used to generate such an accurate frequency reference. Accordingly, known wireless communication devices use more expensive high-precision reference oscillators, and associated precision circuitry, to obtain the necessary accuracy and repeatability.
The precision circuitry in the wireless communication device or mobile handset generally includes a voltage-controlled temperature-compensated crystal oscillator (VC-TCXO). The VC-TCXO provides a reference frequency, which is used by the handset to find, or lock onto the carrier frequency. Depending upon the handset's current temperature, the VC-TCXO adjusts its tuning to maintain a constant reference frequency output. Such VC-TCXO's, however, are relatively expensive, adding significantly to the cost of a handset. Moreover, despite a VC-TCXO's expense, the reference frequency output eventually drifts with age. In addition, VC-TCXOs typically have an indeterminate initial accuracy resulting from manufacturing tolerances that complicates a handset's design.
Further, conventional temperature compensation circuitry for a VC-TCXO has degraded accuracy at temperature extremes. Despite this degraded accuracy, users expect a handset to provide reliable communication that mimics or betters that of a landline phone. To meet expectations, the handset must consistently establish communication in an acceptable time period and do so over a receiver's wide temperature range. For example, a handset is expected to operate in sub-Arctic conditions, and also operate after being subject to the torturous heat of a car dashboard in summer. As a result, it is desirable that a handset VC-TCXO should typically maintain a frequency accuracy of better than about +/−2 parts per million (ppm) over the temperature range −30° C. to +85° C.
As generally described earlier, a VC-TCXO is not stable over time. VC-TCXO modules may drift at approximately 1 ppm per year, yet are often expected to operate for many years. The typical AFC (Automatic Frequency Control) loop used to tune the VC-TCXO modules can cope with offsets of no greater than about +/−4 ppm with respect to its desired reference frequency. Thus, in just a few years, such VC-TCXO modules will have drifted to the point that they may not be able to assist in acquiring or capturing a base station's carrier.
Systems have been developed to account for the aging of a VC-TCXO. For example, U.S. Pat. No. 6,064,270 discloses a handset having a VC-TCXO that, should the base station's beacon carrier escape capture, will perform a random search for the carrier by adjusting the VC-TCXO's reference frequency +/−4 ppm (or some other suitable amount). If the carrier is acquired, the offset is remembered for future use. Although this system may provide acceptable age compensation when coupled with an expensive VC-TCXO, it is not practically implemented with less expensive oscillators having more substantial drift and corresponding large offsets. A system having such large offsets that searched blindly for the carrier frequency would produce delays intolerable to the user. In addition, the search accounts only for the gross offsets encountered during aging and does not address temperature effects, necessitating the continued use of an expensive VC-TCXO.
Accordingly, there is a need in the art for an improved receiver capable of providing a reference frequency that is robust to temperature and aging effects without the use of an expensive VC-TCXO.